The heart is a series of pumps that are carefully controlled by a very special electrical system. This electrical system attempts to regulate the heart rate between 60 and 100 beats per minute (bpm). Abnormally fast heart rates are called tachycardias. As used herein, the term tachycardia means a heartbeat at a rate which is abnormally high and accordingly considered to be dangerous if permitted to continue, or any arrhythmia involving recognizable heartbeat patterns containing repetitions which are in excess of a periodic heartbeat within a safe range. When the ventricular chambers beat too quickly, the arrhythmia (i.e., unusual heart rhythm) is known as ventricular tachycardia (VT). When VT occurs, the ventricles may not be able to fill with enough blood to supply the body with the oxygen rich blood that it needs. Symptoms of VT include feeling faint, sometimes passing out, dizziness, or a pounding in the chest.
Ventricular fibrillation (VF) is a very fast and irregular heartbeat that is caused by abnormal impulses coming from several areas of the heart. These abnormal impulses take over the natural pacemaker function of the sinoatrial (SA) node. The heartbeat is so fast that the heart does not have time to pump enough blood to the brain and body tissue, which may cause unconsciousness, cardiac arrest, or death. Ventricular fibrillation has typically been treated using shock therapy. When a patient's heart is found to be in VF, a jolting electrical pulse, or shock pulse, is delivered to the patient in order to reactivate the electrical signals throughout the heart. The shock pulse may be administered via external defibrillators, or via implantable cardioverter defibrillators (ICDs) configured to deliver such a shock. Patients, however, feel uncomfortable using shock therapy because of the pain and discomfort associated with the shock. This is particularly the case if the shock therapy is applied frequently due to the reoccurring nature of VF.
Ventricular tachycardia, on the other hand, may be controlled by medication in some cases. If medications are not effective, the physician may elect to control the rhythm by electrical methods, such as anti-tachycardia pacing (ATP) therapy.
Generally, VT is distinguished from VF by heart rate. For example, if the heart rate is between 150-210 bpm, the condition is generally considered to be a treatable VT, and therefore ATP therapies may be applied. Heart rates higher than 210 bpm are generally considered to be VF, and are immediately treated with shock. Recent studies, however, have found that ATP therapies can successfully treat heart conditions where the heart rate is above 210 bpm, what was once considered the dividing line between treatable VT and VF. If a more effective method of distinguishing between treatable VT and VF is found, patients can be effectively treated with ATP therapies and avoid the uncomfortable pain of shock therapy.